2 answers

1

The thing about pronouns is to make sure that it is clear who or what we are talking about. Without context, the sentence certainly does seem to need his to be correct, not their. But what about these examples?

Mr. Brown and his son ran a company together. When they decided to file for bankruptcy, they hired a lawyer. The lawyer called their clientsto provide information.

Although I just play here, I feel it it necessary to remind people that Grammarly is just software, and English is an extremely complex language with a huge vocabulary. This complexity means no software today is able to catch 100% of errors. In my own experience, I'd say Grammarly catches about 80% of your errors and still has a fairly high percentage of "false positives."

Your sentence is fairly straightforward, and I'm surprised Grammarly did not catch the problem. You should contact Grammarly Support (they do not read this forum) at the link on the bottom of this page.